234 QUANTITATIVE ANALYSIS. 



until the color of the permanganate no longer completely 

 disappears on shaking. The first permanent faint red color 

 indicates the end-point of the reaction. This is best seen 

 when the flask is placed on a sheet of white paper. If exactly 

 25 cc. are used, then the strength of the solution is correct 

 and each cubic centimeter of the permanganate solution cor- 

 responds to 0.56 mg. of iron. If such an exact agreement is 

 not found, it is best not to change the strength of the solu- 

 tion, but to calculate its value. For example, if, instead of 

 25 cc., 25.6 cc. were used, then the solution is too dilute and 



56 X 25 



1 cc. does not correspond to 0.56 mg. of iron, but to - . c 



Zo.o 



= 0.5468 mg. 



Standardization with Ferrous Ammonium Sulphate. Weigh 

 off exactly 3.924 g. of pure, dry ferrous ammonium sulphate, 

 FeS0 4 ,(NH 4 ) 2 S0 4 + 6H 2 (none of the crystals should be 

 yellow-colored), dissolve in water which has previously been 

 boiled, and dilute the solution with boiled and cold water to 

 one liter. Titrate as with the oxalic acid, but at room tem- 

 perature. 



Titration of the Solution Obtained from the Blood. The 

 titration with the permanganate solution is carried out 

 exactly in the same manner as in standardizing with the 

 ferrous ammonium sulphate solution. Since, however, the 

 zinc contains traces of iron, a check experiment is made with 

 the zinc alone. This procedure does not take into account 

 the slight reducing action of the carbon contained in the 

 zinc. 1 The amount of metallic iron in oxyha3moglobin is 

 approximately 0.4 per cent. The amount of the oxyhse- 

 moglobin is therefore obtained from the amount of iron 



100 

 found by multiplying by g-j = 250. 



1 To avoid this action it is also recommended to dilute the solution 

 obtained exactly to 100 cc.,let settle, and then take out with the pipette 

 50 cc. for the determination. 



